Mens et Manus: Making with Technology Fall 2019

Mens et Manus: Making with Technology Fall 2019

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Mens et Manus

Welcome to Making with Technology!

These notes are posted on our website http://mit.edu/6.a01.

Click “Slides” for the week of October 28.

Also see our “Overview” document (separate tab at top of page)

and our complete “Parts” list.

October 28, 2019

Brushless Motor Project

Brushless Motor technology is the leading motor technology in a

wide range of applications:

• high precision: e.g., computer peripherals,

• low cost: e.g., hand-held power tools, and

• high power: e.g., electric and hybrid automobiles.


We will build a brushless motor using

• a modern microcontroller (Teensy 3.2) and

• parts constructed with modern rapid-prototyping techniques

− laser cutting

− 3D printing

Great project for all skill levels:

• intro level project for students with no previous experience

• many opportunities for optimizations to develop existing skills

How do electric motors work?

Electric motors rotate due to the interaction of magnetic fields that

are fixed to a rotor that rotates and a stator that is stationary.

N

S

S

N

NS

NS

Controlling the Electromagnets

In a brushed motor, currents are switched mechanically using a

commutator, which has parts on both the rotor and stator.


Commutator in an electric drill.

commutator plates

brushbrush

Mechanical switching generates sparks, excess heat, and power loss.


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Sensing rotor position and controlling current flow are separated in

a brushless motor.

Sensing is accomplished with a solid-state device.

Currents are switched electronically.

Hall Effect Sensor

Magnetic fields divert the motion of charged particles.

By

Ix

Ix

fv

e−

Current in x direction results from flow of electrons in −x direction.

Magnetic field B in y direction generates (Lorentz) force f in z

direction,

f = qv ×B

were q is charge on electron and v is it’s velocity.

Lorentz force pushes electrons upward, making conductor more neg-

ative at top than bottom.

Sensing Rotor Position

Two (or more) sensors are usually required to determine the angular

position of the rotor.


We will use electronic switches to activate the electromagnets.

This configuration is called an H-bridge. It consists of two half-

bridges that each control the voltage on one side of the coil.

V

X1H

X1L

X2H

X2L

By opening and closing four switches, one can set the voltage across

a coil to be +V , −V , or zero.


Make a brushless motor:

• using laser-cut or 3D printed parts,

• with electronic sensors and actuators, and

• controlled with a microcontroller.

You can choose

• the configuration of the rotor and stator,

• the number and configuration of coils and magnets,

• the placement of sensors, and

• timing and choreography via a microcontroller.

No previous experience is assumed.

Inrunner and Outrunner Configurations

Electromagnets are (almost) always rigidly attached to base plate.

inrunner outrunner

Inrunner: Disk-shaped rotor spins inside electromagnets.

Outrunner: Annular rotor spins outside electromagnets.


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Parts

We have distributed bags of parts for your motor.

Take some time to familiarize yourselves with the parts.

Motor Design Issues

Attaching the rotor.

The simplest kind of axle is a bolt.

For most purposes that is fine.

Motor Design Issues

Ball bearings are better.

You could use a ball bearing that fits into a 1/2” hole and provides

a freely rotating attachment to a 1/4” shaft.

Motor Design Issues

After assembly.

Rotor

The rotor should firmly attach to the shaft so that the shaft turns

with the rotor.

This can be accomplished using a ”D-shaft” with a corresponding

D-shaped hole in the rotor.

Electromagnets

Coils for electromagnets can be wound on plastic bobbins or by

wrapping wire around a bolt with washers.

An all-metal design better withstands heat produced by windings.

Magnetic force depends on voltage, wire diameter, and number of

turns. We will discuss designing coils at a later session.


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Electromagnets

Coils can be attached to a base plate using angle brackets or inte-

grated into the design of the base plate.

Hall Sensors

Wires can be connected to the hall sensors using a wire-wrapping

tool.

They should be held in place with some sort of fixture, which can

be 3D printed or laser-cut acrylic as shown above.

Finished

Here is an example of a fully assembled motor.

Elevation

Maximum force results when coils line up with rotor plane.

stator

rotor

sha

ft

bearings

spacer

spa

cer

collar

collar

Hall sensor

coil

• Two bearings help stabilize the shaft.

• Two collars hold the rotor assembly together.

• Small-diameter spacers are needed to prevent collar and rotor

from touching outside of bearing.

Electronic Control

The electronic parts are contained on a single printed circuit card.

We will discuss these parts in a later session.

Important Practical Considerations

Magnetic forces diminish rapidly with distance.

0 2 4 6 8 10 12 140

0.25

0.5

0.75

1

no

rma

lize

dfo

rce

distance d [mm]

d

coil

magnet

Force at 2 mm is only half that at 0 mm.

Very little force is exerted at a distance of 20 mm.

→ stator coils can only exert significant force on magnets if distance

to magnet is <10 mm.


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Ferrous cores increase magnetic force but also add static force.

You can hold coils in place with steel screws (highly magnetic) stain-

less steel screws (slightly magnetic) or nylon screws (not magnetic).

Ferrous cores increase magnetic force by concentrating flux (good!).

However ferrous cores are attracted to permanent magnets in rotor

regardless of whether the coil is energized (bad!).

For most designs, the extra force using ferrous cores outweighs dis-

advantages of static forces.


Build a plan for mounting Hall sensors into your conceptual design.

Most designs use one sensor to control each pair of electromagnets.

2 electromagnets → one sensor

4 electromagnets → two sensors

6 electromagnets → three sensors

· · ·


Avoid overly symmetric designs.

What’s wrong with the following design?

NS

S N

NS

SN


Less symmetry: different number of permanent and electromagnets.

NS

SN

NS

SN

NSSN

If permanent magnets have left-right symmetry (as illustrated), we

can still use the horizontal coils will generate torque.


Avoid overly symmetric designs.

What’s wrong with the following placements of Hall sensors?

NS

S N

NS

SN


How about this placement of Hall sensors?

NS

SN

NS

SN

NSSN


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Schedule

Five weeks to design, build, and debug.

Week 1: 10/28 Conceptual (paper) Design

Week 2: 11/4 CAD

11/11 Veterans Day (no class)

Week 3: 11/18 FAB

Week 4: 11/25 Coils and Assembly

Week 5: 12/2 Programming and Debugging

12/9 Motor Presentations

Today: Conceptual (paper) Design

Each student should design their own motor.

However, it’s easier to work with a partner with whom you can

discuss ideas (yours and theirs).

Today: Conceptual (paper) Design

Focus on high-level goals (lasercut vs. 3D printed, geometry, . . .).

We will refine technical aspects in subsequent sessions.

Before leaving today, get a checkoff and upload your sketches:

• Go to our home page: http://mit.edu/6.a01

• Click on “Conceptual (paper) Design”

• Get a Checkoff: discuss your design with a staff member

• Upload pictures of your design sketches.

New Week: CAD

Next week we will use Fusion 360 to make CAD files for your parts.

Download Fusion 360 (free to students) to your laptop before class.

If you’d like to use a loaner laptop, send email to [email protected]

Documents

Mens et Manus: Making with Technology Fall 2019